Active Matrix Organic Light Emitting Diodes (AMOLEDs) are one of the hot spots in the research field of today's flat panel displays. Compared with Liquid Crystal Displays (LCDs), Organic Light Emitting Diodes (OLEDs) have advantages such as low power consumption, a low production cost, self-luminosity, a wide angle of view and a fast response etc. At present, in the display field such as mobile phones, Personal Digital Assistants (PDAs), digital cameras etc., the conventional LCD display screens have began to be replaced by OLED display screens. Pixel driving is the core technical content for AMOLED displays, and has important research significance.
Unlike Thin Film Transistor-Liquid Crystal Displays (TFT-LCDs) that use a stable voltage to control luminance, the OLEDs are driven by a current and require a constant current to control light emission. As shown in FIG. 1, a pixel driving circuit of the conventional AMOLED uses a 2T1C pixel driving circuit. The circuit is only comprised of one Driving Thin Film Transistor (DTFT), a switch thin film transistor T1 and a storage capacitor C. An OLED and the DTFT are connected in series to a driving power supply voltage ELVDD, and a gate of the DTFT is connected to a data line which provides a data signal Vdata through the switch thin film transistor T1. A scanning line is connected to a gate of the switch thin film transistor T1 to gate a row. FIG. 2 illustrates an operation timing diagram of the pixel driving circuit shown in FIG. 1, which shows a timing relationship between a scanning signal provided by the scanning line and a data signal provided by the data line.
When the scanning line gates (i.e., scans) a certain row, in phase t1, the scanning signal Gate(n) is a low level signal, T1 is turned on, and the data signal Vdata is written into the storage capacitor C. After the row is completely scanned, in phase t2, Gate(n) transitions to a high level signal, T1 is turned off, and a gate voltage stored on the storage capacitor C drives the DTFT to generate a current which drives the OLED to emit light.
According to the characteristics of the DTFT, a current passing through the DTFT is
            I      D        =                  1        2            ⁢      μ      ⁢                          ⁢              C        OX            ⁢              W        L            ⁢                        (                                    V              GS                        -                          V              TH                                )                2              ,where VGS is a gate-source voltage of the DTFT, VTH is a threshold voltage of the DTFT, COX is a capacitance of an oxide layer of the DTFT, W and L are a channel width and a channel length of the DTFT respectively, μ is a mobility, and VGS=Vdata-ELVDD. By substituting VGS into the above equation,
      I    D    =            1      2        ⁢          μ      n        ⁢          C      OX        ⁢          W      L        ⁢                  (                              V            data                    -                      ELV            DD                    -                      V            TH                          )            2      is derived. Therefore, in the driving circuit of the OLED, the driving current and the data signal Vdata outputted by the source driving circuit are in a quadratic function relationship.
FIG. 3 illustrates a relationship between a driving current and luminance of an organic light emitting diode. As can be seen from FIG. 3, the luminance of the organic light emitting diode increases as a current density increases, and becomes darker as the current density decreases.
For an OLED display with certain luminance, a current range provided to the OLED is determined. As shown in FIG. 3, when a display in a luminance range of 0˜20000 cd/m2 uses an EFF50 EL material, a driving current range is 0˜37 mA/cm2, and when the display uses an EFF80 EL material with higher efficiency, only 0˜24 mA/cm2 is required. Thus, as the efficiency of the material increases, it is required to reduce the driving current, which reduces power consumption while requiring improved accuracy of the driving current under the same grayscale (8 bits correspond to 256 grayscales).
As can be known from the driving current
      I    D    =            1      2        ⁢          μ      n        ⁢          C      OX        ⁢          W      L        ⁢                  (                              V            data                    -                      ELV            DD                    -                      V            TH                          )            2      of the DTFT, when the driving current range decreases, if an W/L ratio of the DTFT does not change, it is required to reduce a voltage range of Vdata, which requires improved accuracy of a voltage Vdata output by a source driving circuit. The accuracy of the voltage output by the source driving circuit can now achieve 5 mV/grayscale. If the efficiency is then doubled, it needs to achieve 3 mV/grayscale, which has exceeded the process capability of the source driving circuit. Of course, the accuracy of Vdata may also be reduced by reducing the W/L value of the DTFT. However, with the increase of resolution, in a limited pixel space, it is difficult to further increase the channel length of the DTFT.
Therefore, there is a need for an apparatus and method which can improve the accuracy of the driving current and thereby improve the display quality.